This invention relates to dry pulverulent cementicious compositions containing fly ash and to a method of forming the composition.
Over 2xc3x97105 tons of concrete are produced annually in the United States. Any cost effective way of improving the strength, especially the compression strength of concrete is valuable commercially particularly in concrete blocks and in buildings, bridges and other structures in which mixing variations or even weather conditions can have an effect on the pozzolanic reaction and hence the performance of the final concrete product. Accordingly, it is a general objective of the present invention to provide a novel pulverulent composition which is chemically stable, can withstand storage, and is low in cost for use as a portland cement substitute that will increase the compression strength of a concrete end product.
Several processes have been developed for producing aggregate from fly ash. For example, Japanese patent 76015532-B describes the production of a lightweight aggregate from papermaking sludge and fly ash, kraft pulp, lime sludge, waste clay or earth. However, the resulting product had to be calcined at about 1100xc2x0 C., increasing energy requirements and costs.
European patent EP 430 232-A (priority U.S. Ser. No. 443,156) describes ceramic prepared by forming a suspension of clay and water, adding ash and organic matter and dewatering to below 60% by weight, shaping the particles and firing at elevated temperature. The ash, however, is obtained from sewage sludge or paper containing 10% clay and the organic matter is papermill waste or sewage sludge.
Japanese patent 02283678-A describes the production of ceramic products by mixing or kneading silica fines with industrial waste, such as sewage sludge (ash), sludge from water treatment plants, crude refuse incinerated ash, crushed waste soil, fly ash, paper sludge, pulp waste liquid, and sediment from the sea or rivers. The product is molded and fired at an elevated temperature.
These prior products have certain shortcomings. First, they require a fibrous component. Moreover, the energy requirements are substantial due to the need for firing the aggregate in an oven.
U.S. Pat. No. 5,374,308 describes a wet hydraulic cement containing fly ash, about 14%-22% water, boric acid, ground silica and citric acid. By contrast, a major objective of the present invention is to provide a stable dry pulverulent product.
U.S. Pat. No. 5,578,122 describes a method of producing concrete containing fly ash in which gypsum is interground with portland cement for reducing the formation of cracks in a sulfite environment. Significantly, the patent concerns class C fly ash which contains relatively high amounts of calcium mineral matter e.g., 27.3% CaO giving the ash a pozzolanic activity of 92. By contrast, it is an object of the present invention to be able to utilize Class F fly ash that contains little natural calcium oxide mineral but may contain ground limestone and/or quick lime (CaO) in the amount of about 5%. Another method of providing an ash product that resists sulfite damage is described in U.S. Pat. No. 5,928,420 but in that case silica fume is added to Class C fly ash in the amount of about 3%-10% to achieve sulfite resistance.
U.S. Pat. Nos. 6,200,379; 4,210,457; 5,266,111; 5,362,319 (which contain papermill sludge) and U.S. Pat. No. 5,346,549 describe other cementacious products that contain water in which ash is a constituent. Accordingly, there still exists a need for a dry pulverulent ash containing composition as a stable portland cement substitute for improving concrete products and especially the compression strength of concrete products.
In view of these and other shortcomings of the prior art it is an object of the invention to provide a dry pulverulent composition that can be used as a substitute for portland cement to improve the compression strength of concrete products.
Another object of the invention is to provide a pulverulent ash composition of the type described that remains dry, unreacted and stable during shipment and storage.
Another object of the invention is to provide a dry pulverulent ash product that is miscible in all proportions with portland cement, aggregate sand and water to produce an improved concrete product.
Another object of the invention is to provide a dry pulverulent ash composition that can be used in producing concrete products such as concrete blocks, panels and poured concrete which is stronger yet lighter in weight than an otherwise similar concrete product that is made without the use of the invention.
Yet another object of the invention is provide an improved dry pulverulent ash product as a portland cement substitute which is capable of producing concrete products of improved strength although the ash may contain 15% or more sulfur compounds.
It is another object of the invention is to provide a dry pulverulent composition that is inexpensive to produce, is capable of substantially reducing the cost and bulk density of a finished concrete product made therefrom and remains stable during shipment and storage prior to the addition of water and which can be shipped dry in trucks, train cars, in bulk or packaged in paper bags, etc.
Another object is to find a way of maintaining an ash composition in a stable unreacted but potentially chemically reactive state during shipment and storage to thereby assure that the physical properties of the finished concrete are not diminished.
These and other more detailed and specific objects of the invention will be apparent in view of the following specification which illustrates by way of example but a few of the various forms of the present invention that will be apparent to those skilled in the art within the scope of the appended claims.
The present invention provides an environmentally stable pulverulent ash composition formed from fly ash that is useful as a portland cement substitute for improving concrete products. The dry product of the invention is environmentally stable in the sense that it is virtually unaffected by environmental moisture during shipment and storage and is useful as a building material when mixed with portland cement, water, sand and aggregate to form a concrete product. The invention is particularly useful using Class F fly ash which contains only about 5% calcium mineral matter but is also useful using Class C fly ash. The dry, pulverulent ash composition of the present invention improves the physical properties of concrete that is produced therefrom while at the same time reducing its cost.
To form the composition, the moisture content of fly ash is maintained or reduced to less than 5% and preferably less than 1% by weight. With the ash in a dry state, an oxidizer in a dry pulverulent form is then admixed with the ash. The oxidizer raises the pH somewhat when lime is present in the ash and insolublizes sulfite salts contained in the ash. Optionally the composition of the present invention includes a minor amount of portland cement and/or a conditioner such as an ion exchange resin or accelerator in pulverulent form. The composition is uniformly mixed and dried and/or maintained in a dry condition of less than 5.0% moisture during storage. It was found that in this condition it was stable and could later be used as a replacement for portland cement in making any of a variety of formed products such as blocks, slabs, pellets, agglominates, poured concrete and the like which exhibit a compression strength that is substantially greater than equivalent concrete produced from portland cement without the present ash composition. An improvement in compression strength as great as 17% was obtained (ASTM Test C618).
In distinction to prior products, the present invention maintains the ash and oxidizer together in an unreacted state. It was found that although the oxidizer is uniformly and intimately admixed with the ash, no chemical reaction takes place and the product can be considered stable and capable of being stored indefinitely provided the moisture content is reduced or maintained at or below levels set forth herein. If the moisture content is excessive, the strength of the finished concrete product will be reduced, which is of course undesirable.